Currently incoming emails go through a process to delete signatures, entire replies quoted, and basically any noise not part of the actual new message. While this is working well for the Reply Via Email use cases, for the Mailing List Mirror use case this trimming can be damaging.
In a mailing list mirror all content is expected to be mirrored. If actually valid content is trimmed accidentally, then not only the people receiving the emails will miss that content, the category serving as archive will miss that content too. This defeats the purpose of a mirror.
Incoming emails could still be processed trying to identify quoted replies and signatures, but instead of deleted they could be hidden behind a
[...] button, leaving to users the option of expanding the full content.